Search

Benthonic marine species show a wide range of biological reactions to seawater chemical changes through time, from subtle adjustments to extinction. The Early Toarcian Oceanic Anoxic Event (T-OAE) was recently recognized in the Neuquén Basin, Argentina, confirming its global scope. The event was identified chemostratigraphically on the basis of a relative increase in marine organic carbon and a characteristic negative carbon-isotope excursion (δ13C org) in bulk rock and fossil wood in the upper Pliensbachian–lower Toarcian interval in the Arroyo Lapa section (Neuquén). Simultaneously with collection of lithological samples, a high-resolution biostratigraphical survey was carried out, and the scarce benthonic fauna was collected in order to check the biotic response to changing marine geochemical conditions. We present here an analysis of size and abundance data from the T-OAE interval in the Neuquén Basin for the dominant bivalve species, the paper-clam Posidonotis cancellata (Leanza), and relate these data to geochemical proxies (%TOC and δ13C org) obtained at the same locality. The abundance of P. cancellata increased when the rest of the benthos diminished, reaching a maximum at the onset level of the T-OAE, and then decreasing. Size-frequency distributions show a noteworthy lack of juvenile shells. Shell size shows a positive correlation with %TOC in the whole section, though over the T-OAE interval proper, it decreases below the level where the maximum %TOC value is attained and increases above it. Posidonotis cancellata shows features of opportunistic species, such as high tolerance to hypoxia, strong dominance in impoverished environments and a strong dependence on primary productivity, but at the same time had a reproductive strategy more similar to equilibrium species, with relatively low juvenile mortality rates. Several anatomical features suggest adaptation to permanently dysaerobic environments. The species disappeared just before the minimum negative carbon-isotope value was reached; and by the same time the genus became extinct worldwide. Unlabelled Image • P. cancellata size and abundance data from Toarcian relate to OAE geochemical proxies. • Shell size correlates positively with %TOC, probably linked to primary productivity. • Maximum abundance at the onset of the T-OAE, and then decreasing until its extinction. • P. cancellata shows opportunistic species features, but with a low juvenile mortality. • P. cancellata anatomical features suggest adaptation to dysaerobic environments. [ABSTRACT FROM AUTHOR]

Abstract: In a recent paper (Verzi, D.H., Montalvo, C.I., 2008. The oldest South American Cricetidae (Rodentia) and Mustelidae (Carnivora): late Miocene faunal turnover in central Argentina and the Great American Biotic Interchange. Palaeogeography, Palaeoclimatology, Palaeoecology 267, 284–291) the potentially oldest South American Cricetidae (Rodentia) and Mustelidae (Carnivora) have been reported from a Late Miocene (Huayquerian) assemblage located in central Argentina (Caleufú site, La Pampa province). These findings expand the biochron of these important families in South America 3–4Ma earlier than previous records. However, several observations prevent us from accepting the validity of these assumptions. In this paper we discuss the age of the Caleufú assemblage, and the assignment of the GHUNLPam 21722 to Mustelidae. The Caleufú assemblage contains a mixture of Pliocene and Late Miocene faunal elements, and has been assigned to the Huayquerian only due to the “stage of evolution” of some rodents. The lack of isotopic or paleomagnetic data coupled with the isolation of this locality and the absence of a local stratigraphic succession inhibit its correlation with other palaeontological comparable sites and a robust inference about its chronology. The “stage of evolution” of a taxon is not a biostratigraphic tool, thus we cannot rule out the possibility that the Caleufú assemblage has an Early Pliocene (Montehermosan) age. The GHUNLPam 21722, a poorly preserved specimen, shows several characters (e.g., 4 lower incisors, mental foramina below the incisors absent, third lower incisor smaller than the first and second ones) that do not match with Mustelidae (or Carnivora) but instead strongly resemble those observed in didelphimorphian marsupials. The discussed caveats regarding Caleufu assemblage chronology and taxonomy indicate that new and detailed studies are needed before the hypotheses advanced by Verzi and Montalvo can be accepted. [Copyright &y& Elsevier]

In recent decades, southernmost South America (50–56° S) has experienced marked climate change (regional warming, decreased precipitation, reduced snow cover, and increased frequency of heat waves) related to variations in atmospheric circulation over varied timescales. In this paper, we develop oxygen isotopes from tree-ring cellulose (δ18O TRC) as a proxy for climate and atmospheric circulation in order to extend the regional meteorological record to deeper time intervals. Our work focuses on Nothofagus forests in two areas: (i) deciduous N. pumilio forest in the steppe transition zone near Punta Arenas and (ii) humid evergreen N. betuloides forest in the Navarino Island region. To investigate the potential for reconstructing palaeoclimate, δ18O TRC variations were correlated with local climate parameters as well as regional (Amundsen Sea Low, ASL) and hemispheric (Antarctic Oscillation, AAO) atmospheric circulation modes for the last 60 years. N. betuloides δ18O TRC variations show an overall positive trend, indicating isotopic enrichment over the study period, whereas no trend is recorded for the N. pumilio record. The strongest relationships with climate, together with the widest spatial representativeness, occur in the N. betuloides chronology during the growing season (spring to austral summer) and extend spatially from mid to high latitudes. In contrast, the sensitivity of the records is limited to summer months, and spatial correlations are much more limited. In addition, the N. betuloides record shows greater potential for reconstructing local climate features such as soil water (r = − 0.76), wind speed (r = 0.69), and precipitation (r = − 0.66), as well as regional (ASL, r = − 0.80) and hemispheric (AAO, r = 0.77) patterns of extratropical atmospheric circulation. Overall, we conclude that the N. betuloides record represents the most valuable tree-ring climate proxy for southernmost South America over past centuries. • Two annual oxygen isotope (δ18O) records from Nothofagus tree rings in southern South America are established. • Strong climatic signals in δ18O from Nothofagus betuloides and N. pumilio are related to local and regional climatic variations. • Differences in climatic sensitivity among δ18O records are related to species-specific responses and site specificities. • δ18O TRC record of N. betuloides from Navarino Island shows strong relationships with soil water content (r = − 0.76) with regional (ASL, r = − 0.80) and hemispheric (AAO, r = 0.77) extratropical atmospheric circulation. • δ18O TRC records from N. betuloides forests show high potential to reconstruct climate variations during the last 400–500 years. [ABSTRACT FROM AUTHOR]

The megafauna extinction in South America was one of the most profound events, with the loss of 50 genera (~ 83%). Three orders disappeared (Notoungulata, Proboscidea, Litopterna), as did all large xenarthrans, but how this fits into global extinction is uncertain, mainly due to the lack of chronological resolution. In recent years there has been an increase in the number of radiocarbon dating at archeological and paleontological sites, but this information varies greatly from area to area in South America, and few data can be considered to constitute a taxon-date. The timing of the late Pleistocene extinction in the Pampas is poorly established. Most taxa only appear in the biostratigraphic context and many reported 14 C dates do not meet rigorous criteria for accepting dates, including reports suggesting survival of megafauna into the Holocene. In the present paper, we evaluate the published radiocarbon dates in the pampas and present 20 new radiocarbon dates for paleontological sites in order to establish a more accurate “extinction window” for the key taxa. These new dates are sufficiently robust to assess correspondences among last-appearance records of megafauna, first-appearance records of humans, and the Younger Dryas to Holocene climatic transition in the Argentine Pampas. These results highlight the need for greater effort in taxa selection for dating. [ABSTRACT FROM AUTHOR]

Burrowing behavior is an important adaptation of animals that live in arid and semi-arid conditions. In this paper, we describe examples of vertebrate burrows from the Upper Cretaceous (Maastrichtian) Adamantina Formation of the Bauru Basin, Brazil, most likely produced by turtles. The Adamantina Formation preserves abundant and diverse turtle body fossils such as Bauruemys elegans (Testudines: Pleurodira); however turtle burrows have not been previously documented. The newly reported burrows are preserved in fluvial sandstone facies and exposed in sections that partially preserve their three-dimensional geometry. Burrows are simple J-shaped tunnels with a cross-sectional shape that is semicircular (half-dome) with a flattened floor. Such burrows show a partially preserved entrance with an inclined ramp angle (22°), and grooves and ridges up to 1 cm in width preserved along the burrows walls and floor. The architecture and sedimentary facies of the host sandstone body, together with the occurrence of Taenidium barretti and the absence of rhizoliths, suggest that the burrow was excavated by scratch digging into an exposed point bar of a meandering river channel. Based on burrow morphology, dimensions, as well as ridges,and grooves in the walls and floor, we propose that burrows were formed by a chelonian (such as a freshwater turtle) during aestivation. We highlight that these are first examples of turtle burrows reported from the Cretaceous, and their occurrence reinforces the hypothesis that the original function of turtle shells was as an adaptation to fossorial behavior. [Display omitted] • The first freshwater turtle burrows from Cretaceous of South America. • The burrows occur in fluvial point bars of Adamantina Formation. • Evidence of fossorial and aestivation behavior in turtles during Cretaceous. • Results reinforce the hypothesis of fossorial behavior in stem turtles. [ABSTRACT FROM AUTHOR]

Abstract: The Bananal Basin is the most important intracratonic Quaternary sedimentary basin of South America and preserves a good record of the paleohydrological conditions of Central Brazil at the Cerrado–Amazon ecotone. This area is currently a large tropical plain, seasonally flooded by rainfall and water table saturation that can be classified as a seasonal wetland. This paper presents a geological and geomorphological analysis supported with geochronological dating aiming to reconstruct the Pleistocene environmental history of the Bananal Basin. The basin was filled by the sediments sourced by the Araguaia–Mortes fluvial system. The deposits belong to the Araguaia Formation which covers ~106,000km2 of the Bananal Basin. TL and OSL chronologies of the Araguaia Formation show that sedimentation started at least in the Middle Pleistocene (MIS 7) but aggradation and avulsion processes were important in the Lower–Middle Pleniglacial between 70.5ka and 34.0±4.6ka (MIS 4 and 3) and in the Upper Pleniglacial between 24.5±3.1 and 17.2±2.3ka (MIS 2). River avulsions were more intense in the lower Bananal Basin (Bananal Island region). Remarkably, we did not record ages between 17ka and 10ka. Although avulsion seems to have been more active in the past, the abandonment by the Araguaia of a channel belt, now occupied by the underfit Javaés River, indicates that avulsion is still active. The avulsion processes are mainly related to palaeohydrological changes, although present-day seismic activity is also recorded. Additional data is needed to generate a more detailed scenario for the older Middle Pleistocene deposits. The data is remarkable, however, because it is the first Middle Pleistocene record described in a large alluvial tropical system in Brazil. The fluvial record is temporally in agreement with records from other large Brazilian fluvial systems such as the Amazon and Paraná basins. The Bananal plain, however, is unique because of its particular style of sedimentation and geomorphologic processes that generated a mega-scale anabranching-like complex pattern of paleochannels presently occupied by underfit streams. [Copyright &y& Elsevier]

Abstract: We evaluate the opening of the Drake Passage (DP), between Antarctica and South America, and associated changes in ocean circulation as forcing factor for the onset of Antarctic glaciation near the Eocene–Oligocene transition (~34 million years ago). In this paper this hypothesis is tested through sensitivity experiments, using numerical models for the global ocean and atmosphere and for the Antarctic Ice Sheet. The response of the Antarctic continent to the opening of the DP and to the establishment of the Antarctic Circumpolar Current is examined. Two different climate states are reproduced with ocean gateway configurations similar to the Late Eocene and to the Late Oligocene, before and after the opening of the DP. A reduced southward heat flux and a decrease of surface temperature are found in the Antarctic realm when the DP is open. A more massive ice sheet develops on the continent in case of DP open compared to the configuration with closed DP. [Copyright &y& Elsevier]

Interactions between human societies and the environment that they inhabit have been a controversial topic in archaeology for at least the past fifty years. Currently, modern theoretical approaches take this subject as a key issue in their research agenda. This paper presents a review of the main outcomes of several archaeological and multidisciplinary South American projects related to this theme. The case-studies discussed here are all located within arid settings, and can be grouped into three broad geographic areas: Puna (or Altiplano) of northwestern Argentina, Cuyo (west-central Argentina), and southern Patagonia. These regions cover a wide latitudinal range extending from 22° to 52° S. They were selected for comparison due to environmental similarities, and a common record of past climate impacts mainly related to the Medieval Climatic Anomaly (MCA) and the Little Ice Age (LIA). Although the impacts of these climatic changes were locally heterogeneous in their intensity and the quality of the available information is regionally variable, they provide a base-line for comparison and supra-regional integration. The integration of archaeological and palaeoenvironmental data on this broad supra-regional spatial scale allows us to identify interesting historical trajectories associated with particular time periods. As an example, there are some spatial rearrangements of large populations during the MCA, in the three areas. Additionally, there are variable patterns in the changes associated with the different social contexts that impose specific demographic and economic constraints. Finally, this study sets the basis for new questions and provides a guide to the methodological and theoretical issues that we need to address in order to answer them. [Copyright &y& Elsevier]

Abstract: This paper presents an updated, extensive review of glacier fluctuations during the past 1000years in the extratropical Andes of South America between ca. 17° and 55°S. Given the variety of environmental conditions and evidence available for glacier fluctuations across this wide latitudinal range, regional accounts are given for the Desert Andes (∼17°–31°S), the Andes of central Chile and Argentina (31°–36°S), and the North (36°–45°S) and South (45°–55°S) Patagonian Andes. The techniques, dating limitations, and interpretations of the glacier records along this transect are also discussed. Information on glacier fluctuations in the Desert Andes is limited to the 20th century. Documentation on past glacier variations is more abundant in the Central Chilean-Argentinean Andes, but the number of chronologies dealing with glacier fluctuations prior to the 1900s is also limited. Most records indicate that glaciers were generally more extensive prior to the 20th century, with dates of maximum expansion ranging from the 16th to the 19th centuries. The number and extent of glaciers increase significantly in the Patagonian region, where the evidence available for dating glacier variations during the past centuries is more abundant and the dating control for glacier events is generally better than in the northern parts of the study area. For some Patagonian glaciers, maximum Little Ice Age (LIA) or post-LIA advances have been precisely dated by dendro-geomorphological determinations or in situ measurements. However, for most sites, the evidence available is still preliminary and there is considerable variability in the extent and timing of events related to the maximum LIA expansion identified in most areas between the 16th and 19th centuries. Evidence is starting to appear at a growing number of sites for glacier advances during the first half of the past millennium. These events were generally less extensive than the LIA maximum pulses. Despite the occurrence of several post-LIA readvances over the past 100–110years, most areas in the Andes of extratropical South America have experienced a general pattern of glacier recession and significant ice mass losses. The differences in the glacier histories observed at local and regional scales probably reflect the inherent limitations associated with the glacier records and/or the dating techniques used in each case together with the varying dominance of precipitation, temperature and other climatic and non-climatic factors on glacier mass balance and glacier dynamics. These differences indicate that the late Holocene glacier history of southern South America is more complex than commonly assumed. The evidence discussed in this study highlights not only the immense potential for glaciological studies of this region but also a significant need for an increased number of detailed, well-dated records of glacier fluctuations. [Copyright &y& Elsevier]

Abstract: This paper documents the main features of the climate and climate variability over South America, on the basis of instrumental observations gathered during the 20th Century. It should provide a modern reference framework for paleoclimate research in South America, targeting high-resolution proxies over the past few centuries. Several datasets suitable for present-day climate research are first described, highlighting their advantages as well as their limitations. We then provide a basic physical understanding of the mean annual cycle of the precipitation and atmospheric circulation over the continent and the adjacent oceans. In particular, the diversity of precipitation, temperature and wind patterns is interpreted in terms of the long meridional extent of the continent and the disruption of the large-scale circulation caused by the Andes cordillera, the contrasting oceanic boundary conditions and the landmass distribution. Similarly, the intensity and timing of the interannual and interdecadal climatic fluctuations exhibit considerable geographical dependence, as some regions are more influenced by large-scale phenomena rooted in the tropical oceans while others are more influenced by high-latitude phenomena. The impact of these large-scale phenomena over South America is documented by a regression analysis between selected atmospheric indices and the precipitation and temperature fields. We have included a discussion on the seasonality and long-term stability of such impacts, and complemented our general description by an updated review of the literature on climate variability over specific regions. [Copyright &y& Elsevier]

Members of the Order Notoungulata are among the most diverse and common mammals in South America during the Cenozoic. Several lineages within notoungulates (e.g., suborders Typotheria and Toxodontia) show a tendency for increased body sizes and hypsodonty during the last 50 Myr. However, the timing, evolutionary mode, and drivers of such tendencies are not entirely understood. In this paper, we use an extensive database of notoungulate fossil occurrences and body mass and hypsodonty estimates to characterize the evolutionary mode of these two phenotypic traits over time, test the extent to which several factors (e.g., development of open environments in the south of South America) have influenced it through time, and investigate whether large trait values were selected through elevated origination or reduced extinction rates. Our results demonstrate that most of the major notoungulate clades evolved toward larger body sizes (up 1500 kg) and higher tooth crown, from a small and low-crowned tooth ancestor, in a punctuated mode. We also show that body mass and the hypsodonty in typotherians and toxodonts had a coupled evolutionary history. Species sorting was a relevant macroevolutionary process in some notoungulate clades, as taxa with high teeth crown and body mass had lower extinction rates. Finally, the development of the hypsodonty in notoungulates must reflect repeated and quick instances of adaptive responses to the increased availability of volcanic or other terrigenous particles, within the broad context of the SSA Cenozoic Andean mountain building. • Notoungulates increased body sizes and hypsodonty throughout the last 50 Myr. • Typotherians and toxodonts with low-crowned teeth had higher extinction rates. • The body mass and the hypsodonty had a coupled long-term evolution. • The Andean growth appears to be the main driver for the hypsodonty evolution. [ABSTRACT FROM AUTHOR]

Extant turtles exhibit a range of ecological adaptations to terrestrial and pelagic marine habitats. Bone shell microanatomy, and specifically, qualitative and quantitative approaches to shell bone porosity, have been used to infer palaeoecology and habitat. However, such inferences are hindered by the lack of a comprehensive sampling of testudine taxa and, in some instances, of long bones of vertebrates other than turtles. In this paper, we carry out an osteohistological analysis of Testudinata shell bones, including 31 different taxa, both extant and extinct, of known habitats. Costal and peripheral elements of published species and new data presented here are statistically analysed. Although, in some cases, values are congruent with those expected for a particular habitat (e.g. Hydromedusa casamayorensis), this is not true for all taxa examined. Moreover, microanatomical features of several taxa (e.g. Chelonia mydas) are inconsistent with habitat data. Results indicate that, although the microanatomical structure of the shell is related to habitat, this is not the only influence and indeed, in several cases, not the most important factor. • A comprehensive qualitative and quantitative description of shell microanatomy of Testudinata is presented. • Data do not support a correlation between costal bone shell microstructure and habitats. • Paleoecological inferences based on bone microstructure must be taken very cautiously. [ABSTRACT FROM AUTHOR]